1. Field of the Invention
The invention relates to a firing mechanism for a subterranean well perforating gun which may be installed in the wall either with the perforating gun, or subsequent to the placement of the perforating gun in the well, and, in the event that the firing mechanism does not function, can be retrieved from the well and replaced by another firing mechanism in order to effect the firing of the gun.
2. History of the Prior Art
In the completion of modern subterranean wells of substantial depth, it has become a common practice to employ a perforating gun that is run into the well on the bottom of a tubing string. A packer is incorporated in the tubing string and is set in the well casing so as to position the perforating gun adjacent the production formation. This practice has the advantage of permitting a much larger perforating gun to be employed than is possible if the gun were run into the well through a tubing string on a wireline. More importantly, it permits the perforating of the well in the so-called "underbalanced" condition wherein the fluid pressure existing in the tubing string adjacent the formation is substantially less than the anticipated fluid pressure of the production formation after the perforating operation is completed. This permits a relatively high velocity flow of production fluid from the newly formed perforations into the tubing string, thus flushing the perforations of the debris that is commonly associated with the perforating operation.
One negative factor encountered in the mounting of a perforating gun on the bottom of the tubing string is the high cost involved in replacing the gun in the event the gun fails to fire. Obviously, the entire tubing string must be withdrawn from the well, the firing mechanism for the perforating gun repaired or replaced, and then the perforating gun again run into the well on a newly formed tubing string. In a deep well this involved a delay of many hours in completing the well.
There is, therefore, a definitive need for a retrievable and replaceable firing mechanism for a tubing carried perforating gun which may be retrieved from the well and replaced by wireline in the event that the firing mechanism fails to operate.